1. Field of the Invention
This invention relates to new free-flowing and pumpable fluids for use in geological exploration, more particularly well servicing fluids, which contain an oil phase and an aqueous phase using emulsifiers. As a characteristic example of servicing fluids of this type, the invention is described in the following with reference to drilling fluids and drilling muds based thereon. However, the modified auxiliary fluids according to the invention are by no means confined to this particular field of application. Related applications covered by the invention include, for example, spotting fluids, spacers, packer fluids, auxiliary fluids for workover and stimulation and for fracturing.
The use of the new multicomponent mixtures as free-flowing well servicing fluids is of particular importance for the development, particularly the offshore development, of oil and gas occurrences, but is by no means confined to this particular application. The new systems may also be generally used in land-supported drilling operations, for example in geothermal drilling, water drilling, geoscientific drilling and mine drilling.
2. Discussion of Related Art
It is known that drilling fluids for sinking wells in rock and bringing up the rock cuttings are flowable systems thickened to a limited extent which may be assigned to any of the following three classes:
Purely aqueous drilling fluids, oil-based drilling fluids, which are generally used in the form of so-called invert emulsion muds, and water-based o/w emulsions which contain a heterogeneous finely disperse oil phase in the continuous aqueous phase.
Drilling fluids with a continuous oil phase are generally formulated as three-phase systems: oil, water and fine-particle solids. The aqueous phase is heterogeneously and finely dispersed in the continuous oil phase. Several additives are used, including in particular emulsifiers, weighting additives, fluid loss additives, alkali reserves, viscosity regulators and the like. Relevant particulars can be found in the Article by P. A. Boyd et al. entitled "New Base Oil Used in Low-Toxicity Oil Muds" in Journal of Petroleum Technology, 1985, 137 to 142 and in the Article by R. B. Bennett entitled "New Drilling Fluid Technology--Mineral Oil Mud" in Journal of Petroleum Technology, 1984, 975 to 981 and the literature cited therein.
So far as performance properties are concerned, drilling fluids based on aqueous o/w emulsions occupy an intermediate position between purely aqueous systems and oil-based invert muds. Detailed relevant information can be found, for example, in the book by George R. Gray and H. C. H. Darley entitled "Composition and Properties of Oil Well Drilling Fluids", 4th Edition, 1980/81, Gulf Publishing Company, Houston and the extensive scientific and patent literature cited therein and in the manual entitled "Applied Drilling Engineering", Adam T. Borgoyne, Jr. et al., First Printing Society of Petroleum Engineers, Richardson, Tex. (U.S.A).
Even today, oil-based w/o invert systems are undoubtedly the safest fluids, particularly for drilling through water-sensitive clay layers. The continuous oil phase of the w/o invert emulsion forms a continuous semipermeable membrane on the surface of the drilled layers of rock and the cuttings introduced into the drilling fluid so that potential diffusions of water can be direction-controlled. The optimization of the working result achieved by using w/o invert fluids has never been matched by any other type of drilling fluid.
Drilling fluids of the type just mentioned and other well servicing fluids of comparable composition originally used mineral oil fractions as the oil phase. Considerable environmental pollution can thus be caused if, for example, the drilling muds enter the environment either directly or through the drilled rock. Mineral oils are not readily biodegradable and, anaerobically, are virtually non-degradable and, for this reason, may be regarded as long-term pollution. In the last decade in particular, various proposals have been put forward by experts with a view to replacing the mineral oil fractions by ecologically safer and more readily degradable oil phases. Applicants describe possible alternatives for the oil phase, including mixtures of such replacement oils, in a relatively large number of patents and patent applications. The documents in question describe in particular selected oleophilic monocarboxylic acid esters, polycarboxylic acid esters, at least substantially water-insoluble alcohols which flow freely under working conditions, corresponding ethers and selected carbonic acid esters, cf. EP 0 374 671, EP 0 374 672, EP 0 386 638, EP 0 386 636, EP 0 382 070, EP 0 382 071, EP 0 391 252, EP 0 391 251, EP 0 532 570, EP 0 535 074.
However, third parties have also put forward various proposals for alternative oil phases for the field of application in question. For example, the following classes of compounds have been proposed as a replacement for mineral oils in w/o invert muds: acetals, .alpha.-olefins (LAO), poly-.alpha.-olefins (PAO), internal olefins (IO), (oligo)amides, (oligo)imides and (oligo)ketones, cf. EP 0 512 501, EP 0 627 481, GB 2,258,258, U.S. Pat. No. 5,068,041, U.S. Pat. No. 5,189,012 and WO 95/30643 and WO 95/32260.
Today, various alternative oil phases for the field of application targeted by the invention are used in practice. Nevertheless, there is still a need for better balancing of the three key parameters for efficient technical procedure: optimized technological working result, optimized control of the ecological problem area and, finally, optimization of the cost/effectiveness ratio.
The Problem Addressed by the Invention and the Concept of its Technical Solution
The problem addressed by the present invention in its broadest version was to provide a new concept which would enable the overall result to be optimized as required on the basis of the extensive technical knowledge which exists today in the field of application targeted by the present invention. High technical efficiency would be achievable in a reasonable cost/effectiveness ratio and, at the same time, current ecological requirements would be optimally satisfied. This concept is formulated as a broad working principle which, with the aid of expert knowledge, may be varied and thus optimally adapted to the particular application envisaged in numerous specific embodiments.
According to the invention, the technical solution for this broad concept lies in the combination of the following working elements:
The composition of the free-flowing and pumpable water- and oil-based multicomponent mixture ensures that, under the particular in-use conditions, particularly in endangered rock formations within the well, the w/o invert mud is formed with the disperse aqueous phase in the continuous oil phase. PA1 Away from endangered rock formations and, above all, in the working up and elimination of cuttings covered with residues of fluid, phase reversal is possible to form a water-based o/w emulsion. PA1 In the working range and particularly in endangered rock formations, the fluid is present as a w/o invert emulsion which, in known manner, forms the required seal on the surface of the rock in the form of a semipermeable membrane. Optimal well stability can thus be achieved. PA1 At the same time, however, the element of the invention of controlled phase reversal to an o/w emulsion with a continuous aqueous phase and a disperse oil phase, as explained hereinafter, makes the rock cuttings separated from the circulated drilling fluid easier to work up and eliminate, as known to the expert. At least the predominant part of the oil phase present in dispersed form can easily be rinsed off the cuttings either by separate washing or even simply by dumping in seawater in the case of offshore drilling, depending on the eco compatibility of the oil phase. The disperse oil phase floats at least partly in the washing liquid and can be removed or is accessible to simplified aerobic degradation at the surface of the seawater. PA1 a) Determination of the dependence on temperature and the associated phase displacement by experimental testing of the system, more particularly by conductivity measurement. PA1 b) The PIT of the particular system in question can be calculated in advance on the basis of expert knowledge.
The following desirable working results can thus be obtained in combination.
The teaching according to the invention puts this principle of phase inversion into practice by using a working parameter involved in the circulation of the drilling fluid, namely the temperature of the drilling fluid at the particular point of use. Inside the well, the temperatures increase rapidly with increasing depth. The heated drilling fluid containing the hot cuttings also leaves the well with considerably elevated temperatures. By controlling and adjusting predetermined phase reversal temperatures, the desired reversal of the w/o invert phase to the o/w emulsion phase can now be achieved outside the well. Particulars of this phase reversal can be found in the following. The parameter of the phase inversion temperature (PIT) selected in accordance with the invention and thus determined in advance in the particular drilling fluid ensures that the circulated drilling fluid is in the required state of a w/o invert emulsion during the drilling process.
Scientific Background to the Teaching According to the Invention
It is known that emulsifiers or emulsifier systems are used to homogenize immiscible oil/water phases by emulsification. The following general knowledge is relevant in this regard: emulsifiers are compounds which, in their molecular structure, link hydrophilic and lipophilic elements to one another. The choice and extent of the particular units in the emulsifier molecule or emulsifier system in question are often characterized by the HLB value which makes a statement about the hydrophilic/lipophilic balance.
Normally, the emulsifiers or emulsifier systems with--comparatively--strongly hydrophilic components lead to high HLB values and, in practice, generally to the water-based o/w emulsions with a disperse oil phase. Emulsifiers or emulsifier systems with--comparatively--strongly lipophilic components lead to comparatively low HLB values and hence to the w/o invert emulsion with a continuous oil phase and a disperse water phase.
However, this description is highly simplified:
The effect of the emulsifiers or emulsifier systems used can be influenced and hence altered by a number of accompanying factors in the mixture as a whole. In the context of the present invention, known parameters for these modifications include in particular the charging of the aqueous phase with soluble organic and/or inorganic components, for example water-soluble, more particularly polyhydric lower alcohols and/or oligomers thereof, soluble inorganic and/or organic salts, the quantity ratio of emulsifier/emulsifier system to the quantity of oil and, finally, constitutional coordination in the composition of the emulsifier/emulsifier system on the one hand and the molecular structure of the oil phase on the other hand.
A particularly significant parameter in the context of the teaching according to the invention for the specific emulsifier effect in regard to formation of the o/w or w/o emulsion can be the particular temperature of the multicomponent system. At least partly nonionic emulsifiers/emulsifier systems in particular show this effect of pronounced dependence on temperature in mixtures of oil and water phases insoluble in one another.
The above-mentioned system parameter of the phase inversion temperature (PIT) is thus crucially important. In cooperation with the other system parameters mentioned above, the emulsifiers/emulsifier systems used lead to the following emulsion associations:
System temperatures below the PIT form the o/w emulsion while system temperatures above the PIT form the w/o invert emulsion. The system is phase-inverted by shifting the temperature into the other temperature range.
The teaching according to the invention makes use of this and, hence, of the natural variation in this parameter:
In the hot interior of the well, the w/o invert state with a continuous oil phase is guaranteed through the choice of suitable emulsifiers/emulsifier systems in conjunction with the other variables to be taken into account here. In the comparatively cold outside environment, the drilling fluid can be phase-inverted simply by lowering the temperature below the PIT of the system, so that components to be removed are easier to work up. The heat effect which always accompanies the in-rock circulation of the drilling fluid ensures the required high temperature range above the PIT of the system at the hot rock surface and thus renders it neutral to the disperse water component of the drilling fluid in this region.
Before the details of the technical teaching according to the invention are discussed, important relevant literature and expert knowledge of the phenomenon of temperature-dependent phase inversion and the associated parameter of the phase inversion temperature (PIT) are summarized in the following. In the light of this basic knowledge available to the general public, the teaching according to the invention will readily be understood and can be put into practice.
A very detailed account of the phase equilibria of three-component systems of an aqueous phase/oil phase/surfactant (more particularly nonionic emulsifiers/emulsifier systems) can be found in the publication by K. SHINODA and H. KUNEIDA entitled "Phase Properties of Emulsions; PIT and HLB" in "Encyclopedia of Emulsion Technology", 1983, Vol. 1, 337 to 367. The authors also include above all the extensive relevant prior-art literature in their publication, knowledge of the dependence on temperature of the phase inversion of such emulsifier-containing oil/water systems being particularly important for understanding the teaching according to the invention as described in the following. The cited publication of SHINODA et al. discusses in detail this temperature parameter and the effects triggered by its variation in the multiphase system. Above all, however, reference is also made to earlier expert knowledge, cf. for example the earlier publications of K. SHINODA et al.--numbers 7 to 10 in the list of references (loc. cit., pages 366/367). Here SHINODA describes the parameter of the phase inversion temperature (PIT, HLB temperature), the dependence on temperature of the particular system using nonionic emulsifiers being given particular emphasis in the earlier publications of SHINODA et al.--numbers 7 and 8 in the list of references. Free-flowing mixtures based on the three-component systems of oil/water/emulsifier are discussed above all in regard to the dependence of the particular phase equilibrium states established upon the temperature of the multicomponent system. The o/w emulsion state with a disperse oil phase in the continuous water phase which is stable at comparatively low temperatures inverts when the temperature is increased to the phase inversion range (PIT or "middle phase" range). In the event of a further increase in temperature, the multicomponent system inverts to the stable w/o invert state in which the water phase is dispersed in the continuous oil phase.
In his list of references (loc. cit., references 31 and 32), SHINODA refers to earlier works of P. A. WINSOR. In the text of his previously cited publication (pages 344 to 345), the phase equilibrium codes coined by WINSOR, namely WINSOR I, WINSOR III and WINSOR II, are related to the temperature-dependent stable phases o/w--middle phase--w/o: WINSOR I is the stable water-based o/w phase, WINSOR II corresponds to the stable invert phase of the w/o type and WINSOR III denotes the middle phase and thus corresponds to the phase inversion temperature (PIT) range as it is now known both generally and in the context of the teaching according to the invention.
These various phases and, in particular, the (microemulsion) middle phase (WINSOR III) of the particular system may be determined in two ways which it is advisable to combine with one another:
Basically, the following applies in this regard: the phenomenon of phase inversion and the associated phase inversion temperature (PIT) take place in a temperature range which is limited at its lower end with respect to the o/w emulsion state and, at its upper end, with respect to the w/o invert emulsion state. Experimental testing of the particular system, in particular by conductivity measurement at rising and/or falling temperatures, provides figures for the particular PIT lower limit and PIT upper limit--again with the possibility of slight displacements if the conductivity is measured on the one hand at rising temperatures and on the other hand at falling temperatures. To this extent, the phase inversion temperature (PIT) or, better stated, the PIT range agrees with the definition of the previously explained WINSOR III (microemulsion) middle phase, However:
The interval between the PIT lower limit (limitation with respect to o/w) and the PIT upper limit (limitation with respect to w/o invert) is generally a controllable temperature range which is comparatively limited through the choice of suitable emulsifier components or systems. In many cases, the temperature limits in question differ by less than 20 to 30.degree. C. and, more particularly, by no more than 10 to 15.degree. C. The teaching according to the invention can make use of this if the invert fluid--or separated components thereof--is to be clearly converted into the o/w emulsion state. However, for certain embodiments which will be described hereinafter, it can be of interest to use comparatively broad temperature ranges for phase inversion as long as it is ensured that, in the working temperature range in which the drilling fluid is used in the earth's interior, the upper limit of this PIT range (establishment of the w/o invert state) is not only reached, but preferably is comfortably exceeded.
By contrast, calculation of the PIT of the particular system in question according to b) does not lead to exact determination of the above-mentioned temperature limits of the particular PIT range, but instead to a figure lying in the order of magnitude of the PIT range actually occurring in practice. This explains why it can be advisable in practice to combine the phase shift determinations according to a) and b). The following observations apply in this regard:
The experimental conductivity measurement of the system shows optimal conductivity for the water-based o/w fluid, but generally no conductivity for the w/o invert phase. If the conductivity of an emulsion sample is measured at various temperatures (rising and/or falling) in the phase inversion temperature range, the temperature limits between the three ranges mentioned, o/w-middle phase-w/o, can be numerically determined very accurately. The following observations apply in regard to the conductivity or non-existent conductivity of the two limiting ranges: between these two ranges lies the phase inversion temperature range of the particular system of which the lower limit (conductive) and upper limit (non-conductive) can be exactly determined.
This experimental determination of the phase inversion temperature range by conductivity measurements is described in detail in the relevant prior art literature, cf. for example the disclosures of EP 0 354 586 and EP 0 521 981. The o/w emulsions cooled below the phase inversion temperature range were found to have an electrical conductivity of more than 1 mSiemens per cm (mS/cm). A conductivity graph is prepared by slow heating under predetermined program conditions. The temperature range in which conductivity falls to values below 0.1 mS/cm is recorded as the phase inversion temperature range. For the purposes of the teaching according to the invention, a corresponding conductivity graph is also prepared for falling temperatures. In this case, conductivity is determined using a multicomponent mixture which, initially, was heated to temperatures above the phase inversion temperature range and thereafter was cooled in a predetermined manner. The upper and lower limits thus determined for the phase inversion temperature range do not have to be identical with the corresponding values of the previously described determination section with rising temperatures of the multicomponent mixture. In general, however, the respective limits are so close to one another that standardized values can be used for industrial purposes (in particular by averaging the associated limits). However, the practicability of the technical teaching described in detail in the following is guaranteed from the working principles used here even for the case where significant differences in the limits of the phase inversion temperature range are measured on the one hand during determination at rising temperatures and on the other hand during determination at falling temperatures. The components of the multicomponent system have to be adapted to one another in their working parameters and effects in such a way that the working principle according to the invention as described in the foregoing can be put into practice: in the hot interior of the rock borehole, the w/o invert state with continuous oil phase is guaranteed. In the comparatively cold outside environment, the drilling mud can be phase-inverted by lowering the temperature below the PIT so that the components to be separated off are easier to work up.
To reduce the amount of work involved in the experiments, it can be useful to calculate the PIT of the particular multicomponent system. However, the same also applies in particular to potential optimizations in the choice of the emulsifiers or emulsifier systems and their adaptation to the selection and mixing of the aqueous phase on the one hand and the type of oil phase on the other hand in dependence upon other aspects of technical procedure. Relevant expert knowledge has been developed just recently from, basically, totally different fields, more particularly from the production of cosmetics. According to the present invention, this generally valid expert knowledge is now also being applied to the field of geological exploration and to the treatment of existing rock bores with systems containing optimized oil and water phases.
Particular reference is made in this connection to the Article by T H. FORSTER, W. VON RYBINSKI, H. TESMANN and A. WADLE "Calculation of Optimum Emulsifier Mixtures for Phase Inversion Emulsification" in International Journal of Cosmetic Science 16, 84-92 (1994). The Article in question contains a detailed account of how the phase inversion temperature (PIT) range of a given three-component system of an oil phase, a water phase and an emulsifier can be calculated by the CAPICO method (calculation of phase inversion in concentrates) on the basis of the EACN value (equivalent alkane carbon number) characteristic of the oil phase. More particularly, this Article by FORSTER et al. cites important literature for the field targeted by the invention, cf. pages 91 and 92 loc. cit. in conjunction with the actual disclosure of the Article. With the aid of numerous examples, it is shown how the choice and optimization of the emulsifiers/emulsifier systems are accessible to the adjustment of optimal predetermined values for the phase inversion temperature range by the CAPICO method in conjunction with the EACN concept.
On the basis of this fundamental knowledge, mixtures of which the PIT is within the range according to the invention and corresponding mixing ratios can be determined in advance for the components intended for practical use, more particularly the oil phase and associated emulsifiers/emulsifier systems (type and quantity). A first useful basis for carrying out experiments on the lines of method a) is thus established. Over and above calculation of the PIT, it is possible in particular to determine the lower and, above all, upper limits of the range in which the middle phase is formed. The temperature limits above which lies the w/o invert range for the drilling mud in direct contact with the hot inner wall of the well for formation of the continuous semipermeable membrane are thus clearly laid down. In general, it is advisable in practice (see the following explanations of the teaching according to the invention) to select and guarantee this upper limit of the phase inversion temperature range with an adequate safety margin in order to ensure the w/o invert phase required in the hot region.
On the other hand, the temperature should be able at lower values to fall below the w/o invert limit to such an extent that use can be made of the advantages of phase reversal up to the o/w phase and the easier working up of the separated components of the drilling mud to which this generally leads.
To complete the review of relevant expert knowledge, reference is made to the following: in recent years, considerable efforts have been made by researchers to improve so-called enhanced oil recovery by flooding oil-containing rock layers with o/w emulsions containing emulsifiers/emulsifier systems. The goal has been in particular to use corresponding systems for the middle emulsion phase (WINSOR III) within the formation. This will immediately become clear from the opposing objective deviating from the teaching according to the invention: optimization of the o/w-w/o equilibrium to form the microemulsion phase in the multicomponent system leads to an increase in the effectiveness of the washing process required in flooding and hence to an increase in the washing out of the oil phase from the rock formation. It is crucially significant in this regard that, by virtue of the microemulsion state, the unwanted blockage of pores in the rock by relatively large oil droplets can be safely prevented.
The objective of the invention is the opposite of this step of enhanced oil recovery by flooding:
The object of the teaching according to the invention in using w/o invert emulsions is to seal the porous surface of rock formations in the well by the continuous oil layer. At the same time, however, the invention seeks to achieve easier disposal of the drilling mud or rather components thereof by phase inversion outside the well.